Direct arterial pressure was recorded continuously over 24 hours in 18 totally unrestricted people (six normotensives, four untreated hypertensives, and eight treated hypertensives). There was an almost equal fall of about 20% in both systolic and diastolic blood pressure during sleep in the three groups when compared with their waking pressures. This fall in pressure was greater than that observed previously in patients sleeping in a laboratory or hospital. Furthermore, it suggests that hypertensive subjects do not have a higher centrally-induced vasoactive component and that hypotensive drugs do not alter the pattern of blood pressure behaviour induced by sleep.